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| United States Patent |
5,766,797
|
|
Crespi
, et al.
|
June 16, 1998
|
Electrolyte for LI/SVO batteries
Abstract
A lithium/silver vanadium oxide cell is disclosed that includes an improved
electrolyte composition having the solvents propylene carbonate and
1,2-dimethoxyethane, and an additional third solvent that reduces the
solubility of the composition of the silver vanadium cathode material.
Preferably, the third solvent is a dialkyl carbonate such as dimethyl
carbonate, diethyl carbonate or ethylmethyl carbonate. The improved
electrolyte composition reduces the build up of resistance in the cell
during cell discharge, and can affect the cell's performance in
implantable cardiac defibrillator applications.
| Inventors:
|
Crespi; Ann M. (Minneapolis, MN);
Chen; Kevin (New Brighton, MN)
|
| Assignee:
|
Medtronic, Inc. (Minneapolis, MN)
|
| Appl. No.:
|
757220 |
| Filed:
|
November 27, 1996 |
| Current U.S. Class: |
429/332; 429/219 |
| Intern'l Class: |
H01M 006/14 |
| Field of Search: |
429/194,196,197,218,219
|
References Cited
U.S. Patent Documents
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|
| 3873369 | Mar., 1975 | Kamenski | 429/194.
|
| 3877983 | Apr., 1975 | Hovsepian | 429/194.
|
| 3877988 | Apr., 1975 | Dey et al. | 429/194.
|
| 3884723 | May., 1975 | Wuttke | 429/162.
|
| 3904432 | Sep., 1975 | Dey et al. | 429/194.
|
| 3930885 | Jan., 1976 | Dey | 429/116.
|
| 3945848 | Mar., 1976 | Dey et al. | 429/198.
|
| 3981748 | Sep., 1976 | Margalit | 429/164.
|
| 4016338 | Apr., 1977 | Lauck | 429/149.
|
| 4028138 | Jun., 1977 | Dey | 29/623.
|
| 4053692 | Oct., 1977 | Dey | 429/171.
|
| 4057679 | Nov., 1977 | Dey | 429/194.
|
| 4084045 | Apr., 1978 | Kegelman | 429/194.
|
| 4091188 | May., 1978 | Dey | 429/174.
|
| 4113929 | Sep., 1978 | Margalit | 429/194.
|
| 4139680 | Feb., 1979 | Schlaikjer | 429/50.
|
| 4158722 | Jun., 1979 | Lauck et al. | 429/194.
|
| 4168351 | Sep., 1979 | Taylor | 429/48.
|
| 4177329 | Dec., 1979 | Dey et al. | 429/101.
|
| 4184017 | Jan., 1980 | Kelsey et al. | 429/197.
|
| 4238552 | Dec., 1980 | Dey et al. | 429/101.
|
| 4252876 | Feb., 1981 | Koch | 429/197.
|
| 4423124 | Dec., 1983 | Dey | 429/194.
|
| 4430399 | Feb., 1984 | Schoolcraft | 429/218.
|
| 4470939 | Sep., 1984 | Schoolcraft | 264/104.
|
| 4556613 | Dec., 1985 | Taylor et al. | 429/101.
|
| 4752541 | Jun., 1988 | Fauklner et al. | 429/101.
|
| 4874680 | Oct., 1989 | Koshiba et al. | 429/197.
|
| 4983476 | Jan., 1991 | Slane et al. | 429/197.
|
| 5079109 | Jan., 1992 | Takami et al. | 429/197.
|
| 5162178 | Nov., 1992 | Ohsawa et al. | 429/218.
|
| 5180574 | Jan., 1993 | Von Sacken | 423/594.
|
| 5294498 | Mar., 1994 | Omaru et al. | 429/122.
|
| 5296318 | Mar., 1994 | Gozdz et al. | 429/192.
|
| 5310553 | May., 1994 | Simon et al. | 429/212.
|
| 5358805 | Oct., 1994 | Fujimoto et al. | 429/218.
|
| 5370949 | Dec., 1994 | Davidson et al. | 429/224.
|
| 5378560 | Jan., 1995 | Tomiyama | 429/217.
|
| 5385794 | Jan., 1995 | Yokoyama et al. | 429/194.
|
| 5395711 | Mar., 1995 | Tahara et al. | 429/197.
|
| 5401599 | Mar., 1995 | Tahara et al. | 429/218.
|
Primary Examiner: Skapars; Anthony
Attorney, Agent or Firm: Woods; Thomas F., Patton; Harold R.
Claims
We claim:
1. An electrochemical cell, comprising:
(a) an anode comprising lithium;
(b) a cathode comprising a silver vanadium oxide material;
(c) an electrolyte comprising a solute and a solvent mixture of propylene
carbonate and 1,2-dimethoxyethane, the solvent mixture further comprising
a third solvent that is a dialkyl carbonate selected from the group
consisting of dimethyl carbonate, diethyl carbonate and ethylmethyl
carbonate, the third solvent being homogeneously miscible with the solvent
mixture of propylene carbonate and 1,2-dimethoxyethane, the third solvent
reducing the solubility of the silver vanadium oxide material.
2. An electrochemical cell as in claim 1 wherein the solvent mixture
comprises about 20-30 percent by volume of propylene carbonate, 20-30
percent by volume of dimethyl carbonate and 40-60 percent by volume of
1,2-dimethoxyethane.
3. An electrochemical cell as in claim 1, wherein the solute is selected
from the group consisting of lithium hexafluoroarsenate (LiAsF6), lithium
hexafluorophosphate (LiPF.sub.6), lithium imide (Li(CF.sub.3
SO.sub.2).sub.2 N), lithium tris(trifluoromethane sulfonate) carbide
((Li(CF.sub.3 SO.sub.2).sub.3 C), lithium tetrafluoroborate (LiBF.sub.4),
lithium triflate (LiCF.sub.3 SO.sub.3), and lithium perchlorate
(LiClO.sub.4).
4. In an electrochemical cell having a first quantity of lithium anode
material and a second quantity of silver vanadium oxide cathode material,
the cell producing a voltage discharge curve having a second voltage
plateau at about 2.6 volts and having an electrolyte composition in
contact with at least a portion of the first quantity of lithium anode
material and in contact with at least a portion of the second quantity of
silver vanadium oxide cathode material, the electrolyte composition
including a solute and a solvent mixture of propylene carbonate and
1,2-dimethoxyethane, the solvent mixture having a dialkyl carbonate as a
third solvent.
5. An electrochemical cell as in claim 4, wherein the dialkyl carbonate is
selected from the group consisting of dimethyl carbonate, diethyl
carbonate, and ethylmethyl carbonate.
6. An electrochemical cell as in claim 5, wherein the solvent mixture
comprises about 20-30 percent by volume of propylene carbonate, 20-30
percent by volume of dimethyl carbonate and 40-60 percent by volume of
1,2-dimethoxyethane.
7. An electrochemical cell as in claim 4, wherein the solute is selected
from the group consisting of lithium hexafluoroarsenate (LiAsF6), lithium
hexafluorophosphate (LiPF.sub.6), lithium imide (Li(CF.sub.3
SO.sub.2).sub.2 N), lithium tris(trifluoromethane sulfonate) carbide
((Li(CF.sub.3 SO.sub.2).sub.3 C), lithium tetrafluoroborate (LiBF.sub.4),
lithium triflate (LiCF.sub.3 SO.sub.3), and lithium perchlorate
(LiClO.sub.4).
8. An electrochemical cell as in claim 5, wherein the solute is selected
from the group consisting of lithium hexafluoroarsenate (LiAsF6), lithium
hexafluorophosphate (LiPF.sub.6), lithium imide (Li(CF.sub.3
SO.sub.2).sub.2 N), lithium tris(trifluoromethane sulfonate) carbide
((Li(CF.sub.3 SO.sub.2).sub.3 C), lithium tetrafluoroborate (LiBF.sub.4),
lithium triflate (LiCF.sub.3 SO.sub.3), and lithium perchlorate
(LiClO.sub.4).
9. An electrochemical cell as in claim 4, wherein the solute is selected
from the group consisting of lithium hexafluoroarsenate (LiAsF6), lithium
hexafluorophosphate (LiPF.sub.6), lithium imide (Li(CF.sub.3
SO.sub.2).sub.2 N), lithium tris(trifluoromethane sulfonate) carbide
((Li(CF.sub.3 SO.sub.2).sub.3 C), lithium tetrafluoroborate (LiBF.sub.4),
lithium triflate (LiCF.sub.3 SO.sub.3), and lithium perchlorate
(LiClO.sub.4).
Description
BACKGROUND OF THE INVENTION
This invention relates to electrochemical cells having a lithium anode and
more particularly to a primary lithium electrochemical cell adapted for
high reliability and high rates of current discharge.
Implantable cardiac defibrillators are used to treat patients suffering
from ventricular fibrillation, a chaotic heart rhythm that can quickly
result in death if not corrected. In operation, the defibrillator device
continuously monitors the electrical activity of the heart of the patient,
detects ventricular fibrillation, and in response to that detection,
delivers appropriate shocks to restore a normal heart rhythm. Shocks as
large as 30-35 joules may be needed. Shocks are delivered from capacitors
capable of providing that amount of energy to the patient in a fraction of
a second. To provide timely therapy to the patient after the detection of
ventricular fibrillation, it is necessary to charge the capacitors with
the required amount of energy in only a few seconds. Thus, the power
source must have a high rate capability to provide the necessary charge to
the capacitors, possess low self-discharge to have a useful life of many
months, and must be highly reliable to provide an urgently needed therapy
whenever necessary. In addition, since cardiac defibrillators are
implanted, the battery must be able to supply energy from a minimum
packaged volume.
One battery suitable for defibrillator use includes silver vanadium oxide
as a cathode material as disclosed in U.S. Pat. Nos. 4,310,609 or
4,391,729 issued to Liang et al or U.S. Pat. No. 5,221,453 issued to
Crespi. The cathode materials described in the foregoing Liang and Crespi
patents can find application in the batteries or cells disclosed in U.S.
Pat. Nos. 5,458,997; 5,312,458; 5,298,349; 5,250,373; 5,147,737;
5,114,811; 5,114,810; 4,964,877; and 4,830,840. All the foregoing patents
are hereby incorporated by reference herein in their respective
entireties.
As disclosed in some of the foregoing patents, the anode material of the
battery is lithium and the reactive cathode material is silver vanadium
oxide. The electrolyte for a lithium battery or cell is a liquid organic
type which comprises a lithium salt in combination with an organic
solvent.
Organic solvents known for use in lithium cells can be, for example,
3-methyl-2-oxazolidone, sulfolane, tetrahydrofuran, methyl-substituted
tetrahydrofuran, 1,3-dioxolane, propylene carbonate (PC), ethylene
carbonate, gamma-butyrolactone, ethylene glycol sulfite, dimethylsulfite,
dimethyl sulfoxide or mixtures thereof and also, for example, low
viscosity cosolvents such as tetrahydrofuran (THF), methyl-substituted
tetrahydrofuran(Met-THF), dioxolane (DIOX), dimethoxyethane (DME),
dimethyl isoxazole (DMI), diethyl carbonate(DEC), ethylene glycol sulfite
(EGS), dioxane, dimethyl sulfite (DMS) or the like. The ionizing solute
for lithium cells can be a simple or double salt or mixtures thereof, as
for example, LiCF.sub.3 SO.sub.3, LiBF.sub.4, LiAsF.sub.6, LiPF.sub.6 and
LiCIO.sub.4 which produce an ionically conductive solution when dissolved
in one or more solvents. An organic solvent composition commonly used for
lithium/silver vanadium oxide cells has been a mixture of propylene
carbonate and 1,2-dimethoxyethane in a 50/50 ratio.
The selection of the particular solvent components and acceptable ratios of
the solvent components can prove to be a difficult task even if each
component is individually well known. Typically a solvent component may be
selected for its dielectric constant, for its capabilities as a solvent
for the particular solute material, for its viscosity or for other
properties which may be unique to a particular cell. For example, since
1,2-dimethoxyethane has a low viscosity and a low dielectric constant, it
is commonly mixed with another polar aprotic solvent having a higher
dielectric constant (e.g.,propylene carbonate, ethylene carbonate, or
gamma-butyrolactone) for use in practical lithium cells and batteries.
Such a solvent mixture possesses better properties for the ionization of
lithium salts and wetting of the electrode and separator surfaces than
either of the component solvents alone.
Electrolytes have also been indicated to be suitable for use in lithium
cells with three solvent components. For example, U.S. Pat. No. 4,129,691
issued to Broussely, and hereby incorporated by reference herein in its
entirety, discloses an electrolyte for use in lithium/cupric oxide or
lithium/ferrous sulfide primary cells which is made from a mixture of
three organic solvents and an alkaline solute. The first solvent is chosen
to have a dielectric constant equal to or greater than 35 (e.g. propylene
carbonate), the second solvent is a linear polyether with its ether
functional groups in the gamma position (e.g. 1,2-dimethoxyethane) and the
third solvent has a high solvation power for dissolving large quantities
of the alkaline salt (e.g. 1,3-dioxolane).
In lithium/silver vanadium oxide cells, it has been noted that the cell
tends to increase in resistance in a roughly time-dependent manner after
the battery is discharged to a second voltage plateau. This means that on
long-term discharge, these cells can develop high resistance that impairs
their ability to charge the capacitors of a defibrillator in a timely
manner and therefore renders much of the capacity of the cell unavailable
for long term use in an implantable defibrillator. Further, the end of
service determination in these cells is complicated by the variable nature
of the resistance buildup. In an experiment which substituted ethylene
carbonate for propylene carbonate, it was found that the irreversible
resistance was much worse with the ethylene carbonate. This is contrary to
expectation since ethylene carbonate has a higher dielectric constant than
propylene carbonate so that the solvent with ethylene carbonate should
have reduced resistance for the cell. Accordingly, it is believed that the
solubility of the silver vanadium oxide cathode material in the
electrolyte solvent contributes to the build-up of,resistance over time.
It is therefore an object of the present invention to provide a high
current rate capability lithium/silver vanadium oxide battery having a
reduced resistance at the second voltage plateau.
It is also an object of the present invention to provide an electrolyte for
a lithium/silver vanadium oxide battery which provides improved discharge
characteristics for the battery.
SUMMARY OF THE INVENTION
These and other objects are accomplished by the electrochemical cell and
electrode assembly of the present invention. We have discovered a
lithium/silver vanadium oxide cell which includes an improved electrolyte
composition which includes the solvents propylene carbonate and
1,2-dimethoxyethane and a third solvent which reduces solubility of the
composition for the silver vanadium cathode material. Preferably, the
third solvent is a dialkyl carbonate such as dimethyl carbonate, diethyl
carbonate, or ethylmethyl carbonate. Most preferably, the electrolyte
composition includes about 20-30 percent by volume of propylene carbonate,
20-30 percent by volume of dimethyl carbonate and 40-60 percent by volume
of 1,2-dimethoxyethane. Some preferred solutes for the electrolyte of the
present invention include lithium hexafluoroarsenate (LiAsF6), lithium
hexafluorophosphate (LiPF.sub.6), lithium imide (Li(CF.sub.3
SO.sub.2).sub.2 N), lithium tris(trifluoromethane sulfonate) carbide
((Li(CF.sub.3 SO.sub.2).sub.3 C), lithium tetrafluoroborate (LiBF.sub.4),
lithium triflate (LiCF.sub.3 SO.sub.3), and lithium perchlorate
(LiCIO.sub.4).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view showing the insertion of an
electrode assembly into a battery case together with insulator materials.
FIG. 2 is a partial cut-away perspective view of a completed battery
showing the connections of the tabs of the electrode with the case
elements.
FIG. 3 is a partial cut-away perspective view of the isolation components
for a battery.
FIG. 4 is an exploded perspective view showing the application of the
insulator and case top to the case and electrode assembly of FIG. 1.
FIG. 5 is a graph showing a discharge curve for a lithium/silver vanadium
oxide battery with a second voltage plateau.
FIG. 6 shows a graph of hexafluoroarsenate molarity versus resistivity for
a 20% EC (ethylene carbonate), 20% DMC (dimethyl carbonate), and 60% DME
(1,2-dimethoxyethane) electrolyte solution of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a lithium/silver vanadium oxide cell
which includes an improved electrolyte composition which includes the
solvents propylene carbonate and 1,2-dimethoxyethane and a third solvent
which reduces solubility of the composition for the silver vanadium
cathode material.
Although a variety of battery configurations and constructions are possible
for lithium/silver vanadium oxide batteries, a coiled or wrapped
configuration will be discussed herein as an exemplar of a conventional
lithium/silver vanadium oxide battery for use with the invention. The
invention is of course applicable to any configuration and construction.
Figure shows an exemplary battery construction. It can be seen that a
coiled electrode assembly comprised of elongated anode and cathode
subassemblies including anode material (lithium) or cathode material
(silver vanadium oxide--SVO which may also include PTFE binder, carbon
black and graphite) pressed onto a metal current collector (Ni, Ti, etc.)
and enveloped with a separator of microporous material such as
polyethylene, polypropylene or the like are overlaid with respect to each
other and coiled up. Connector tabs may be included within the electrode
assembly for making electrical connection thereto.
Assembly of the electrode assembly 120 into a battery is shown in FIGS.
1-4. In FIG. 1, a coil insulator 200 is placed onto the electrode assembly
120. The coil insulator includes a notch 202 to accommodate anode
connector tab 22 and slits 204, 206 208 to accommodate anode connector tab
20, and cathode connector tabs 70, 72 respectively. The electrode assembly
120 is also inserted into an insulative case liner 210. The case liner 210
preferably extends at its top edge above the edge of the electrode
assembly 120 in order to provide an overlap with other insulative
elements. If so, it may include a notch 211 on one side in order to allow
the easy connection of the anode connector tabs 20, 22 to the case 220.
The coil insulator 200 and case liner 210 are preferably made from a
polyolefin polymer or a fluoropolymer such as PTFE or PETFE. The electrode
assembly 120 and case liner 210 are then inserted into a prismatic case
220, preferably made of stainless steel or titanium.
In FIG. 4, a case cover 230 and a pin insulator 240 are shown along with
the electrode assembly 120 and prismatic case 220. The case cover 230 has
a glassed in feedthrough 232 and feedthrough pin 233 extending through an
aperture in the case cover 230 that has a bend 234 which is intended to
place the feedthrough 232 in alignment with the cathode connector tabs 70,
72. The case cover 230 also has a fill port 236. The case cover 230 is
made from stainless steel and the feedthrough pin 233 is preferably formed
of niobium, tantalum or molybdenum. The pin insulator 240 has an aperture
242 leading into a raised portion 244 which receives the feedthrough pin
233 and insulates the feedthrough pin 233 from contact with the case cover
230.
In combination with one side of the coil insulator 200, which is
immediately below the pin insulator 240, the raised portion forms a
chamber which isolates the cathode connections. Additional insulation in
the form of tubing or a coating (not shown) may also be included on the
feedthrough pin 233 and feedthrough 232 at locations which will not be
welded to further insulate the feedthrough pin 233 and feedthrough 232 and
also an additional cover insulator (not shown) could be applied to the
underside of the case cover 230 to provide additional insulation for the
case cover 230. The feedthrough pin 233 is welded to the cathode connector
tabs 70, 72 (as shown in FIG. 2) and the anode connector tabs 20, 22 are
bent into an "L" shape and are welded to the side of the case 220 thereby
making the metal case 220 one terminal or contact for the battery (i.e. a
case negative design). The feedthrough pin 233 is then inserted through a
split (not shown) in the pin insulator 240 until it projects through the
aperture 242 of the pin insulator 240. The electrode assembly 120 may be
out of the case 220 during some of the welding and bending operations. The
case cover 230 is then welded to the case 220 to seal the electrode
assembly 120 in the case.
FIG. 3 shows the isolation components of the battery in greater detail. A
cover insulator 245 is adapted to fit under the case cover 230 with an
aperture 246 to accommodate the feedthrough 232 and feedthrough pin 233
and a cut-away portion 247 to accommodate the fill port 236. The cover
insulator 245 is applied to the underside of the case cover 230. A
feedthrough insulator 250 then slides over the feedthrough pin 233 and
over the feedthrough 232 into contact with the cover insulator 245. Once
the feedthrough insulator 250 is in place, a tubular insulator 255 is
slipped over the feedthrough pin 233 until it contacts the feedthrough
insulator 250. The feedthrough pin 233 is then bent into its desired
configuration for connection with cathode connector tabs 70, 72 as shown
in FIG. 4.
The pin insulator 240 is shown with a split 241 which extends from the edge
of the pin insulator 240 to the aperture 242. Again, the pin insulator 240
has an aperture 242 leading into a raised portion 244 or recess which
receives the feedthrough pin 233 and the tubular insulator 255 over the
feedthrough pin and insulates the feedthrough pin 233 from contact with
the case cover 230 at the point where the feedthrough pin is welded to the
cathode connector tabs 70, 72. The split 241 allows the pin insulator 240
to be placed on the feedthrough pin 233 after the feedthrough pin has been
welded to the cathode tabs 70, 72. The tubular insulator 255 therefore
extends through the aperture 242, thereby preventing any discontinuity in
the isolation of the cathode connector tabs 70, 72 and feedthrough pin 233
from elements at anode potential.
A coil insulator 202a is shown with a notch 202 to accommodate anode
connector tab 22 and slits 204, 206 to accommodate anode connector tab 20,
and cathode connector tab 70 respectively. A notch 208a is also provided
to accommodate cathode connector tab 72 in place of the slit 208 shown in
FIG. 1. The electrode assembly 120 is also inserted into an insulative
case liner 210. All of the case isolation components including the cover
insulator 245, the feedthrough insulator 250, the tubular insulator 255,
the pin insulator 240, the coil insulator 202a and the case liner 210 are
molded or extruded self-supporting polymeric parts preferably made from a
polyolefin polymer or a fluoropolymer such as PTFE or PETFE.
The result of this insulator configuration is that the cathode connections
are thoroughly isolated from the portions of the battery at anode
potential and that the feedthrough connection is thoroughly isolated from
stray particles of material from the cathode and from lithium particles
that may form during discharge of the battery.
An appropriate electrolyte solution is introduced through the fill port 236
and the fill port 236 is then sealed. The electrolyte solution can be an
alkali metal salt in an organic solvent such as a lithium salt (i.e., 1.0M
LiCIO.sub.4 or LiAsF.sub.6) in the solvent composition of the present
invention. A mixture of propylene carbonate and dimethoxyethane together
with a third solvent component which reduces solubility of the composition
for the silver vanadium cathode material is used. The sealing process (not
shown) may include, for example, making a first seal by pressing a plug
into the aperture of the fill port 236 and making a second seal by welding
a cap or disc over the fill port 236. Material utilized for leak checking
hermetic seals may be included between the first and second seals.
As already indicated, such batteries are generally known in the art and
used to power defibrillators. As shown in FIG. 5, the open-circuit voltage
of such lithium/silver vanadium oxide cells have two voltage plateaus, a
first voltage plateau 300 at about 3.2 v and a second voltage plateau 310
at about 2.6 v, with two sloping regions 315, 320. The cells of FIG. 5 are
balanced with sufficient lithium and electrolyte to discharge the cathode
to completion. With a silver vanadium oxide formula of Ag.sub.2 V.sub.4
O.sub.11, it has been calculated that about 6.67 equivalents of lithium
are required to completely discharge one equivalent of silver vanadium
oxide.
Thus, in one embodiment of the present invention, an electrochemical cell
has a first quantity of lithium anode material and a second quantity of
silver vanadium oxide cathode material, and the cell produces a voltage
discharge curve having a second voltage plateau at about 2.6 volts. The
cell of such an embodiment of the present invention is further
characterized in having an electrolyte composition in contact with at
least a portion of the first quantity of lithium anode material and in
contact with at least a portion of the second quantity of silver vanadium
oxide cathode material, and the electrolyte composition includes a solute
and a solvent mixture of propylene carbonate and 1,2-dimethoxyethane, the
solvent mixture having a dialkyl carbonate as a third solvent.
In the present invention, the improved electrolyte composition includes the
solvents propylene carbonate and 1,2-dimethoxyethane together with a third
solvent which reduces solubility of the composition for the silver
vanadium cathode material. That solvent must be a polar, aprotic solvent
which is homogeneously miscible with the propylene carbonate and
1,2-dimethoxyethane components, and which is nonreactive with the battery
components. Preferably, the third solvent is a dialkyl carbonate such as
dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate.
Longer chain alkyl groups may also find application as or in the third
solvent. For example, longer chain alkyl groups formed using propyl groups
(C.sub.3 H.sub.7 groups) or butyl groups (C.sub.4 H.sub.9), or
combinations thereof, may find application as the third solvent, or as
components thereof. Some cyclic, aromatic or aliphatic carbonates may also
find application as or in the third solvent such as
dicyclopentylcarbonates.
Most preferably, the electrolyte composition includes about 20-30 percent
by volume of propylene carbonate, 20-30 percent by volume of dimethyl
carbonate and 40-60 percent by volume of 1,2-dimethoxyethane. Also, most
preferably, the concentration of 1,2-dimethoxyethane should be less than
50% in order to minimize gas formation since high levels of
1,2-dimethoxyethane in the presence of residual moisture can promote the
formation of methane which is detrimental to battery performance.
The solute is preferably lithium hexafluoroarsenate (LiAsF6), lithium
hexafluorophosphate (LiPF.sub.6), lithium imide (Li(CF.sub.3
SO.sub.2).sub.2 N), lithium tris(trifluoromethane sulfonate) carbide
((Li(CF.sub.3 SO.sub.2).sub.3 C), lithium tetrafluoroborate (LiBF.sub.4),
lithium triflate (LiCF.sub.3 SO.sub.3), or lithium perchlorate
(LiCIO.sub.4). Lithium hexafluoroarsenate is the most preferred solute for
the electrolyte at a one molar concentration.
Table 1 below shows experimental resistivity data obtained with a
conventional, prior art electrolyte and electrolytes of the present
invention. The conventional, prior art electrolyte comprised one molar
lithium hexafluoroarsenate in 50/50 volume percent mixture of propylene
carbonate and 1,2-dimethoxyethane. The electrolytes of the present
invention comprised varying amounts of propylene carbonate (PC), dimethyl
carbonate (DMC), diethyl carbonate (DEC), 1,2-dimethoxyethane (DSE),
varying concentrations of lithium hexafluoroarsenate, as set forth in
Table 1 below. Electrolyte resistivities were measured at room temperature
using an conventional conductivity probe connected to a Hewlett Packard
4192A Impedance Analyzer.
TABLE 1
______________________________________
Measured Electrolyte Resistivities
Electrolyte Composition
Resistivity (Ohm-centimeters)
______________________________________
conventional electrolyte
82.2
1 M LiAsF.sub.6 in 25% PC, 25% DMC,
65.1
and 50% DME
1 M LiAsF.sub.6 in 40% PC, 20% DMC,
69.1
and 40% DME
1 M LiAsF.sub.6 in 20% PC, 40% DMC,
63.4
and 40% DME
1 M LiAsF.sub.6 in 20% PC, 20% DMC,
61.5
and 60% DME
0.8 M LiAsF.sub.6 in 40% PC, 20% DMC,
69.5
and 40% DME
0.8 M LiAsF.sub.6 in 20% PC, 40% DMC,
65.6
and 40% DME
0.8 M LiASF.sub.6 in 20% PC, 20% DMC,
64.1
and 60% DME
1.2 M LiAsF.sub.6 in 40% PC, 20% DMC,
69.2
and 40% DME
1.2 M LiAsF.sub.6 in 20% PC, 40% DMC,
63.6
and 40% DME
1.2 M LiAsF.sub.6 in 20% PC, 20% DMC,
63.3
and 60% DME
______________________________________
Table 1 shows that the electrolytes of the present invention provide
enhanced conductivity in respect of a known, prior art electrolyte.
Enhanced conductivity in a battery generally results in improved rate
capability because ionic transport in the electrolyte is quicker and more
efficient.
FIG. 6 shows a graph of hexafluoroarsenate molarity versus resistivity for
a 20% EC, 20% DMC, and 60% DME electrolyte solution of the present
invention. Electrolyte resistivities in this experiment were measured at
room temperature using an conventional conductivity probe connected to a
Hewlett Packard 4192A Impedance Analyzer. A resistivity minimum is seen to
occur at about 1.1M LiAsF.sub.6, indicating that advantageous results can
be obtained in a primary lithium battery containing an electrolyte of the
present invention.
It will be appreciated by those skilled in the art that while the invention
has been described above in connection with particular embodiments and
examples, the invention is not necessarily so limited and that numerous
other embodiments, examples, uses, modifications and departures from the
embodiments, examples and uses may be made without departing from the
scope of the present invention.
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